The vertical screw at the bottom of the float bowl is the main mixture adjustment, and should be open one and a half turns from fully screwed in, as a starting point just so it will run and you can adjust it accurately. The screw on the top of the carburetor with the spring under its head is the idle mixture adjustment, which you also open one and a half turns. The screw next to the idle mixture adjustment is the main jet, and it should be fully screwed in or it will leak. See diagram:

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